Image forming systems include ink deposition units to form images on substrate. The ink deposition units could be such as inkjet printheads that eject droplets of ink and deposit the droplets on the substrate. Substrates could include different materials such as paper, plastics, glass, stone, and others. The printing is done in raster mode. Raster mode means that the images are printed or reproduced by scanning each and every line of a grid and depositing an ink droplet at a desired coordinate on the line/grid. At the end of the line, the printhead is moved back to the beginning of the scan and incrementally shifted to print another line. Printing in the raster printing mode is performed by accessing each of the points of the grid regardless if there is an ink droplet to be deposited at this point or not. Relative movement between the substrate and the printhead facilitates placing the ejected ink droplet on any point of the grid.
Some of the images to be printed could cover all surface of the substrate. Layout of other images could cover a segment of the substrate and continue on another segment of the substrate. There could be no printed image between the different image segments. Some images, for example, entrance doors or windshield windows of a car are printed on the borders of a substrate only. The desired image quality determines the print resolution and the grid pitch. High quality images are printed at high printing resolution and take much more time than images of similar size printed at a lower printing resolution. Generally, the printing time depends on the printing resolution, the size of the image and the geometry of the image. In large format industrial printing systems where the printing is performed on substrates with sizes of 3000×4000 mm or 5000×6000 mm the printing could take hours regardless of the image layout.
Printing time is reduced by assembling individual inkjet printhead modules into ink deposition units printing a larger then a single printhead module swath. Each of the individual inkjet printhead modules has a characteristic signature, since ink-ejecting orifices located along the module eject different sizes of ink droplets producing visible artifacts in the printed image. In some printing applications this effect is mitigated by what is termed multipass printing. In multipass printing the ink deposition unit moves back and forth and passes a number of times over the printed swath and different ink ejecting orifices deposit ink droplets contributing to the same segment of the image. In other more critical printing applications individual inkjet printhead modules could be rotated to mitigate swath butting artifacts, but the printing itself is performed in the same raster printing mode and no printing time is saved.